(1) Field of the Invention
The present invention relates to a method and test for field sample arsenic speciation in aqueous solutions, in particular, arsenic speciation in drinking water. In particular, the method and test uses in series three columns wherein the first column removes interfering cations from water, the second column removes dimethylarsenate (DMA) from the water, and the third column removes As(V) (collectively H3AsO4/H2AsO4−/HAsO42−/AsO43−) and monomethylarsonate (MMA) while As(III) (collectively H3AsO3/H2AsO32−/HAsO32−/AsO33−) remains in the water effluent. The bound arsenic species are separately eluted from the columns in a laboratory and then each of the eluants and the effluent are tested for the arsenic species, preferably using graphite furnace atomic absorption spectroscopy.
(2) Description of Related Art
Arsenic (As) contamination in natural waters is a worldwide problem that has become the subject of considerable international attention. Long-term ingestion of drinking water contaminated with arsenic has been associated with health problems including skin, bladder and lung cancers, skin lesions, cardiovascular effects, and anemia (Tseng et al., J. Nat. Cancer Inst., 1968. 40, 453–463; Wu et al., Am. J. Epidem., 1989, 130, 1123–1132; Chen and Wang, Cancer Res., 1990, 5, 5470–5474; Guo et al., Epidemiol., 1997, 8, 545–550; Hopenhayn-Rich et al., Intl. J. Epidemiol., 1998, 2, 561–569). Widespread health problems, including what is termed “blackfoot disease”, have been observed in Bangladesh following a program to replace contaminated surface water supplies with shallow wells. Arsenic levels as high as 1820 μg/L have been reported in some wells in Taiwan (Tseng et al., J. Nat. Cancer Inst., 1968. 40, 453–463). Drinking water regulations for maximum allowable levels of arsenic vary by country across the range of 1 μg/L to 50 μg/L, the U.S. having recently lowered the standard to 10 μg/L (World Health Organization (WHO), Drinking Water Guidelines and Standard, 1996; Federal Register 66,14:6976–7066). In most cases, the source of the arsenic is natural, resulting from the dissolution of arsenic-containing minerals in the aquifer (Kim et al., Environ. Sci. Technol., 2000, 34, 3094–3100; Simon, et al., Am. Mineral., 1999, 84, 1071–1079).
Investigations to date have focused largely on total arsenic levels in water supplies, but it is now understood that toxicity varies with the chemical form of the arsenic (National Research Council (NRC), Arsenic in Drinking Water. Washington: National Academy Press, 2001; Jain and Ali, Water Res., 2000, 34, 4304–4312). The common inorganic arsenic species in water include As(III) forms (H3AsO3, H2AsO3−, HAsO32−) and As(V) forms (H3AsO4, H2AsO4−, HAsO42−, AsO43−) (Purnedu and Sharma, Water Res., 2002, 36, 4916–4926; Jeckel, In Arsenic in the Environment Part I; Nriagu, ED. John Wiley & Sons, New York, 1994, pp. 119–132). In addition, dissolved arsenic can occur as the organic forms dimethylarsinic acid/dimethylarsinate ((CH3)2AsO(OH)) (DMAA/DMA), and monomethylarsonic acid/monomethylarsonate ((CH3)AsO(OH)2) (MMAA/MMA). As(III) is reported to be 25 to 60 times as toxic as As(V) and several hundred times as toxic as methylated arsenicals (Korte and Fernando, Cri. Rev. Environ. Control, 1991, 21, 1–39). A number of other organic forms can be found, but these are generally considered to be non-toxic (Shiomi, In Arsenic in the Environment Part I; Nriagu, ED. John Wiley & Sons, New York, 1994, pp. 261–293; Philips, Aquat. Toxicol., 1990, 16, 151–186).
Total arsenic measurement in environmental matrices is relatively straightforward, with the principal methodologies based on flame (FAAS), hydride generation (HGAAS) or graphite furnace atomic absorption (GFAAS). Arsenic speciation has been more problematic and a number of separation techniques have been employed to resolve the various species. Separation approaches reported in the literature include methods based on sequential volatilization (Braman et al., Anal. Chem., 1977, 49, 621–625; Carvalho and Hercules, Anal. Chem., 1978, 50, 2030–2034; Heinrichs and Keltsch, Anal. Chem., 1982, 54, 1211–1214; Tesfalidet, S., Irgum, K. Anal. Chem., 1988, 60, 2031–2035; Galban et al., Spectr. Acta PartB-Atomic Spectr., 1993, 48, 53–63; Moller and Scholz, Anal. Proceedings, 1995, 32, 495–497; Frankenberger, Soil Biol. Biochem., 1998, 30, 269–274; Anderson et al., Analyst, 1986, 111, 1443; Lopez-Molinero et al., J. App. Spectr., 2001, 55, 1277–1282; Lopez-Molinero et al., Mikrochimica Acta, 1999, 131, 225–230), liquid-liquid extraction (Kalyanaraman and Khopkar, Talanta, 1977, 24, 63–65; Chakraborti et al., Anal. Chim. Acta, 1980, 120, 121–127; Puttermans et al., Anal. Chim. Acta, 1983, 149, 123–128; Bohr and Meckel, Fres. J. Anal. Chem., 1992, 342, 370–375), arsine generation (Narsito and Agterdenbos, Anal. Chim. Acta, 1987, 197, 315–321; Masscheleyn et al., Environ. Sci. Technol., 1991, 25, 1414–1419; Masscheleyn et al.,. Environ. Sci. Technol.,1991, 20, 96–100; Michel et al., Mikrochimica Acta, 1992, 109, 35–38; Howard and Comber, Mikrochimica Acta, 1992, 109, 27–33; Cabreros Pinillos et al., J. Anal. Chim. Acta, 1995, 300, 321–327; Burguera and Burguera, Talanta, 1997, 44, 1581–1604), ion exchange (Henry and Thorpe, Anal. Chem., 1980, 52, 80–83; Pacey and Ford, Talanta, 1981, 28, 935–938; Ficklin, Talanta, 1983, 30, 371–373; Aggett and Kadwani, Analyst, 1983, 108, 1495–1499; Gómez et al., Fres. J. Anal. Chem.,1997, 357, 844–849; Edwards et al., J.A.W.W.A,. 1998, 90, 104–113; Miller et al., Water Res., 2000, 34, 1397–1400; Le et al., Environ. Sci. Technol., 2000, 34, 2342–2347; Kim, Bull. Environ. Contam. Toxicol., 2001, 67, 46–51), ion-pair high-performance liquid chromatography (Grabinski, Anal.Chem., 1981, 53, 966–968; Ding et al., J. Chromat. A, 1995, 694, 425–432; Yalcin, Talanta, 1998, 47, 787–796; Le and Ma, Anal. Chem., 1998, 70, 1926–1933; Mattusch and Wennrich, Anal. Chem., 1998, 70, 3649–3655; Spuznar et al., Spectrochim. Acta B, 2000, 55, 779–793), and reverse-phase high-performance liquid chromatography (Brinckman et al., J. Chrom., 1980, 191, 31–46; Tye et al., Anal. Chim. Acta, 1985, 169, 195–200; Ebdon et al., Analyst, 1988, 113, 1159–1165; Bohari et al., Anal. Atomic Spec., 2001, 16, 774–778; Martin et al., J. Chrom.B: Biom. Appl., 1995, 66, 101–109; Le et al., Anal.Chem., 1996, 68, 4501–4506; Larsen, Spect. Acta Part B 1998, 53, 253–265; Londesborough et al., J. Fres. Anal. Chem., 1999, 363, 577–581).
The problem with using most of the above techniques to assess exposure to various arsenic species is that they are laboratory based and the distribution of species in a collected sample is not necessarily stable. For example, it has been shown that As(III) can be oxidized to As(V) during storage even when water samples preserved with hydrochloric acid and other chemicals (Borho and Wilderer, J. Water Supply Res. Technol.—Agua, 1997, 46, 138–143; Volke and Merkel, Acta Hydrochim., 1999, 27, 230–238). The arsenic separation methods reported in the literature that might be carried out on-site are limited to separation of As(III) and As(V) (Ficklin, Talanta, 1983, 30, 371–373; Edwards et al., J.A.W.W.A,. 1998, 90, 104–113; Kim, Bull. Environ. Contam. Toxicol., 2001, 67, 46–51; Russeva et al., Fres. J. Anal. Chem., 1993, 347, 320–323). Thus, there remains a need for a method capable of determining the distribution of the most toxic arsenic species (As(III), As(V), MMA, and DMA) at the point of exposure, at concentration levels of interest in drinking water.
Elevated arsenic concentrations are most frequently observed in groundwater supplies where it may exist as inorganic arsenate/arsenous acid (As(III)), or as arsenate/arsenic acids (As(V)). A number of organic forms can also be found in aquatic systems with monomethylarsonic acid ((CH3AsO(OH)2 or MMA) and dimethylarsinic acid ((CH3)2AsO(OH) or DMA) being the most prevalent.
These different arsenic species exhibit significantly different toxicities: As(III) is reported to be 25 to 60 times as toxic as As(V) and several hundred times as toxic as methylated arsenicals (Kim et al., Environ. Sci. Technol., 2000, 34, 3094–3100). Therefore, a standard based only on total arsenic concentrations may actually represent a broad range of risk factors. These species also have different physical-chemical properties, which may require different water treatment strategies. Thus, while knowledge of total arsenic concentrations in drinking water may be reasonable from a regulatory perspective, advances in risk assessment and risk management will likely depend on knowledge of the individual arsenic species.
Speciation analysis of an element in water samples (Lund, Fres. J. Anal. Chem., 1990, 337, 557–564; Ure, Mikrochim. Acta, 1991, II, 49–57) involves the use of analytical methods that can provide information about the physicochemical forms of the element, which together make up its total concentration in the sample. The individual physicochemical forms may include particulate matter and dissolved forms such as simple organic species, organic complexes and the element adsorbed on a variety of colloidal particles (Florence, Analyst, 1986, III, 489–524). Schroeder (Trends Anal. Chem., 1989, 8, 339–342) distinguishes physical speciation, which involves differentiation of the physical size or the physical properties of the element, and chemical speciation, which refers to the differentiation among the various chemical forms. Thus, speciation can be defined as the occurrence of different forms, chemical and/or physical, of an element in the real samples.
The speciation of arsenic in the environment is of great significance due to the different toxicity levels exhibited by the various species present in environmental and clinical samples (WHO, Arsenic in Drinking Water, 2001, Vol. 2001; Jain, Water Res., 2000, 34, 4304–4312). The more common arsenic species are arsenate As(III), arsenate As(V), arsenious acids (H3SO4, H2AsO4−, HAsO42−), arsenic acids (H3AsO4, H2AsO4−, HAsO42−), dimethylarsinate (DMA), monomethylarsonate (MMA), arsenobetaine (AB) and arsenocholine (AC). These forms illustrate the various oxidation states that arsenic commonly exhibits (−III, 0, III, V) and the consequently complexity of its chemistry in the environment.
Arsenic contamination in natural water is a worldwide problem and has become a challenge for the world scientists. Arsenic speciation is important because As(III) is found to be more water soluble and more mobile in the environment (Woolson, In Topics in Environmental Health, Biological and Environmental Effects of Arsenic; Fowler, Ed.; Elsevier: New York, 1983; pp. 51–139). The degree of toxicity of arsenic species is inversely proportional to the rate of excretion from the body and can be shown as follows: arsine>As (III)>As (V)>MMA>DMA (Puttemans and Massart, Analytical Chimica Acta 1982, 141, 225; Russeva and Havezov, Bulgarian Chem. Comm., 1993, 26, 228–238). Data on the differences in toxicity between As(III) and As(V) on human beings are very limited.
In spite of the numerous analytical approaches for arsenic speciation in the last few years, there still remains the need for a sensitive method, which allows determining effectively, and simultaneously, the four most important arsenic species (As(III), As(V), MMA, and DMA) in different matrixes. The approaches that have been developed for speciation studies of arsenic have their own advantages and limitations. However, research efforts, are still needed to develop inexpensive, rapid, sensitive, and reproducible methodologies for arsenic species capable of working in the range of drinking water limits.